Multi-carrier modulation (MCM) communication systems are known by a variety of other names including orthogonal frequency division multiplexing (OFDM) and discrete multi-tone (DMT), and MCM has been employed in several applications such as high definition television (HDTV), digital audio broadcasting (DAB) and digital subscriber loop (DSL), systems. An MCM signal is a summation of a significant number of sub-carrier signals. Consequently, as in audio communication, the amplitude of the MCM signal bas a Gaussian distribution, which has a large peak amplitude to average amplitude ratio (PAPR).
Communication systems that use DMT are known to be robust to frequency selective fading and therefore, DMT can be used reliably for high data rate transmissions. However, the high peak amplitudes in a DMT signal can cause several disadvantages. One disadvantage is, the large dynamic range of the DMT signal causes a digital to analog converter in a DMT transmitter to saturate causing the DMT signal to be clipped. Clipping causes severe non-linear distortion to the transmitted DMT signal, which cannot be corrected in a corresponding DMT receiver. In addition, clipping introduces clipping noise that further degrades the transmitted DMT signal.
In order to overcome clipping, the maximum level of the digital to analog converter can be set at a very high value to accommodate the peak values. However, for a fixed transmit power level, an expensive digital to analog converter having a high resolution will be required. Alternatively, the resolution of the digital to analogue converter (DAC) can be reduced to reduce cost. However, this results in a larger step size, which increases quantization noise, thus degrading the transmitted DMT signal.
Another disadvantage is the wide dynamic range imposes the need for a power amplifier with a large dynamic range or back off. Such power amplifiers are expensive and are not power efficient, which limits the use of DMT systems that utilize such power amplifiers to, for example, non-portable applications.
Another method of reducing PAPR disclosed in a publication tided “Understanding Digital Subscriber Line Technology” by T. Starr, J. M. Cioffi and P. J. Silverman, published by Printice-Hall, 1999, teaches tone reduction. Here, a predetermined number of sub-carriers inject symbols that reduce the PAPR of the MCM signal, and an iterative algorithm teaches which symbols are injected. However, to reduce the PAPR significantly, up to 20% of the sub-carriers are required to inject the symbols, leaving fewer sub-carriers for carrying information. In addition, this method is complex as it requires iterative minimization of non-linear functions and computation of several fast Fourier transforms (FFTs).
Other known methods of reducing PAPR revolve around managing clipping, however such methods can themselves cause secondary clipping.